I occasionally see customers who are taking backups from their PXC clusters that complain that the cluster “stalls” during the backup. As I wrote about in a previous blog post, often these stalls are really just Flow Control. But why would a backup cause Flow control?
Most backups I know of (even Percona XtraBackup) take a FLUSH TABLES WITH READ LOCK (FTWRL) at some point in the backup process. This can be disabled in XtraBackup (in certain circumstances), but it is enabled by default.
If you go to your active cluster right now an execute a …
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